1. Field of the Invention
The present invention relates to coherent detection, and more specifically, it relates to a low cost, compact, and temperature-insensitive optical hybrid.
2. Description of Related Art
Since the late 1990s, the transport capacities of ultra-long haul and long-haul fiber-optic communication systems have been significantly increased by the introduction of the erbium-doped fibre amplifier (EDFA), dense wavelength division multiplexing (DWDM), dispersion compensation, and forward error correction (FEC) technologies. For fiber-optic communication systems utilizing such technologies, the universal on/off-keying (OOK) modulation format in conjunction with direct detection methods have been sufficient to address data rates up to 10 Gb/s per channel.
In order to economically extend the reach and data capacity beyond such legacy systems and into next-generation networks, several technological advancements must take place, including but not limited to, 1) adoption of a differential phase-shift keying (DPSK) modulation format, as opposed to OOK; 2) developments in optical coherent detection; and 3) progress in adaptive electrical equalization technology. In combination, these technologies will boost a signal's robustness and spectral efficiency against noise and transmission impairments.
Such crucial strides in optical signal technology are no longer theoretical possibilities but are feasible solutions in present-day optical networking technology. The path for an optical coherent system has already been paved by 1) the deployment of DPSK modulated systems by Tier-1 network providers; and 2) the increased computational capacity and speed of electronic DSP circuits in receivers, which provides an efficient adaptive electrical equalization solution to the costly and difficult optical phase-lock loop. These advances coupled with a commercially feasible optical hybrid solution would likely give pause to Tier-1 providers and carriers to reassess their earlier rationales for not adopting and implementing an optical coherent detection scheme. Perhaps with such advances, optical networks will begin to realize the benefits already recognized in microwave and RF transmission systems for extending capacity and repeaterless transmission distances through coherent detection.
The commercial feasibility of a coherent system for optical signal transmission was first investigated around 1990 as a means to improve a receiver's sensitivity. In contrast to existing optical direct-detection system technology, an optical coherent detection scheme would detect not only an optical signal's amplitude but phase and polarization as well. With an optical coherent detection system's increased detection capability and spectral efficiency, more data can be transmitted within the same optical bandwidth. More over, because coherent detection allows an optical signal's phase and polarization to be detected and therefore measured and processed, transmission impairments which previously presented challenges to accurate data reception, can, in theory, be mitigated electronically when an optical signal is converted into the electronic domain. However, the technology never gained commercial traction because the implementation and benefits of an optical coherent system could not be realized by existing systems and technologies.
Implementing a coherent detection system in optical networks requires 1) a method to stabilize frequency difference between a transmitter and receiver within close tolerances; 2) the capability to minimize or mitigate frequency chirp or other signal inhibiting noise; and 3) an availability of an “optical mixer” to properly combine the signal and the local amplifying light source or local oscillator (LO). These technologies were not available in the 1990s. A further setback to the adoption and commercialization of an optical coherent system was the introduction of the EDFA, an alternative low cost solution to the sensitivity issue.
Notwithstanding the myriad challenges, an optical coherent system (also referred to as “Coherent Light Wave”) remains a holy grail of sorts to the optical community because of its advantages over traditional detection technologies. Coherent Light Wave provides an increase of receiver sensitivity by 15 to 20 dB compared to incoherent systems, therefore, permitting longer transmission distances (up to an additional 100 km near 1.55 μm in fiber). This enhancement is particularly significant for space based laser communications where a fiber-based solution similar to the EDFA is not available. It is compatible with complex modulation formats such as DPSK or DQPSK. Concurrent detection of a light signal's amplitude, phase and polarization allow more detailed information to be conveyed and extracted, thereby increasing tolerance to network impairments, such as chromatic dispersion, and improving system performance. Better rejection of interference from adjacent channels in DWDM systems allows more channels to be packed within the transmission band. Linear transformation of a received optical signal to an electrical signal can then be analyzed using modern DSP technology and it is suitable for secured communications.
There is a growing economic and technical rationale for adoption of a coherent optical system now. Six-port hybrid devices have been used for microwave and millimetre-wave detection systems since the mid-1990s and are a key component for coherent receivers. In principle, the six-port device consists of linear dividers and combiners interconnected in such a way that four different vectorial additions of a reference signal (LO) and the signal to be detected are obtained. The levels of the four output signals are detected by balanced receivers. By applying suitable baseband signal processing algorithms, the amplitude and phase of the unknown signal can be determined.
For optical coherent detection, a six-port 90° optical hybrid should mix the incoming signal with the four quadratural states associated with the reference signal in the complex-field space. The optical hybrid should then deliver the four light signals to two pairs of balanced detectors. Let S(t) and R denote the two inputs to the optical hybrid and
            S      ⁡              (        t        )              +          R      ⁢                          ⁢              exp        ⁡                  [                      j            ⁡                          (                                                π                  2                                ⁢                n                            )                                ]                      ,with n=0, 1, 2 and 3, represent the four outputs from it Using the PSK modulation and phase-diversity homodyne receiver as an illustration, one can write the following expression for the signal power to be received by the four detectors:
                    P        n            ⁡              (        t        )              ∝                  P        S            +              P        R            +              2        ⁢                                            P              S                        ⁢                          P              R                                      ⁢                  cos          ⁡                      [                                                            θ                  S                                ⁡                                  (                  t                  )                                            +                                                θ                  C                                ⁡                                  (                  t                  )                                            -                                                π                  2                                ⁢                n                                      ]                                ,      n    =    0    ,            …      ⁢                          ⁢      3        ;  where PS and PR are the signal and reference power, respectively, θS(t) the signal phase modulation, and θC(t) the carrier phase relative to the LO phase. With proper subtractions, the two photocurrents fed to the TIA's can be expressed asIBD1∝√{square root over (PSPR)} cos[θS(t)+θC(t)];IBD2∝√{square root over (PSPR)} sin[θS(t)+θC(t)];encompassing the amplitude and phase information of the optical signal. Accordingly, the average electrical signal power is amplified by a factor of 4PR/PS. Following this linear transformation the signals are electronically filtered, amplified, digitized and then processed. Compared to a two-port optical hybrid, the additional two outputs have eliminated the intensity fluctuation from the reference source (LO).
An optical coherent receiver requires that the polarization state of the signal and reference beam be the same. This is not a gating item as various schemes or equipment are available to decompose and control the polarization state of the beams before they enter the optical hybrid. Further, certain polarization controllers can be used to provide additional security functionality for optical coherent systems, preventing third parties from tapping information or data streams by implementing polarization scrambling and coding techniques.
For laboratory purposes, a 90° optical hybrid has traditionally been constructed using two 50/50-beam splitters and two beam combiners, plus one 90° phase shifter. These optical hybrids can be implemented using all-fiber or planar waveguide technologies; however, both methods have their respective drawbacks. Both technologies require sophisticated temperature control circuits to sustain precise optical path-length difference in order to maintain an accurate optical phase at the outputs. In addition, fiber-based devices are inherently bulky and are unstable with respect to mechanical shock and vibration; whereas, waveguide-based products suffer from high insertion loss, high polarization dependence and manufacturing yield issues. Waveguide-based products are also not flexible for customization and require substantial capital resources to set up.
Accordingly, a low-cost, temperature insensitive and vibration/shock resistant optical hybrid and method of operating same is desirable and such is provided by the present invention.